In particular, the method in accordance with the invention relates to controlling pressure in a high-pressure region of a common rail injection system. In the case of this type of injection system, the process of generating pressure that is required to inject the fuel is decoupled from the process of injecting fuel into the combustion chamber of the internal combustion engine. The fuel is conveyed independently of the injection cycle of the internal combustion engine by way of a high-pressure pump into a storage unit, the so-called rail. The rail is connected to the fuel injectors of the engine by way of high-pressure lines. The injectors are actuated in an electrical manner by way of a control and said injectors ensure that fuel is injected from the high-pressure region into the combustion chamber of the engine.
The common rail system is divided primarily into a low-pressure region and a high-pressure region. The low pressure region comprises inter alia a fuel tank and fuel lines. The high-pressure region comprises inter alia a high pressure pump, a common rail, fuel injectors and high pressure lines.
A typical piezo servo injector comprises as its main components a holding body, an injection nozzle, a control valve and a control chamber. By way of a pressure change in the control chamber of the injector, the valve needle of the injection nozzle can be raised or lowered and the injection nozzle can consequently be opened or closed respectively. The pressure in the control chamber can be adjusted by way of controlling the control valve. The control chamber is connected on the one hand by way of the control valve to the low-pressure region and on the other hand by way of an inlet restrictor to the high-pressure region of the injection system.
In general, the manner in which a piezo servo injector functions can be divided into four states. In the idle state, the injector is not actuated. The control valve is closed and the pressure from the high-pressure region builds up in the control chamber by virtue of the in-flow of fuel from the rail. The force that is exerted on the valve needle by virtue of the pressure in the control chamber urges the valve needle head into the valve seat of the injection nozzle; the injection nozzle is closed in this position. As the injection process commences, the piezo actuator of the injector is charged by way of a charging signal and the control valve is consequently opened. The fuel can flow out of the control chamber into the low-pressure region. The inlet restrictor ensures that the pressure reduction in the control chamber cannot be compensated for immediately by means of the fuel pressure in the rail. The pressure reduction in the control chamber ensures as a consequence that the closing force acting of the needle valve reduces and the injection nozzle is opened. In the open state, the fuel is injected from the rail into the combustion chamber of an associated cylinder. As the injection process is terminated, the piezo actuator is discharged by way of a discharge signal and the control valve is consequently closed. As a result, the pressure in the control chamber is equalized to that of the fuel pressure prevailing in the rail. The increase in pressure in the control chamber ensures that the closing force that is acting on the needle valve in turn increases and the injection nozzle is consequently closed.
In order to meet the rising demands to reduce the pollutant emissions from internal combustion engines, solutions for fuel injection systems can be found that entail new requirements for maintaining system performance. One measure for reducing CO2 emissions is for example to reduce the injection leakage of a fuel injector. However, this is encumbered with the problem that lack of leakage-tightness in a fuel injector can lead to the undesired injection of fuel into the combustion chamber of a cylinder. The undesired injected fuel is not combusted properly in the cylinder and this leads as a result to an increase in pollutant emissions.
A known measure for reducing injector leakage is the reduction of pressure in the common rail system of the internal combustion engine. However, the additional components that are required for this purpose, such as for example an actuator for controlling the pressure in the rail, lead to a considerable increase in the costs of the system of the internal combustion engine.
In the case of current systems, a pressure reduction occurs in the rail by means of a continuous injector leakage from a high-pressure region into a low-pressure region of the internal combustion engine. As a consequence, it is ensured that the customer requirements with regard to a pressure reduction gradient in the high-pressure region are met. The use of injectors that have a very low or no continuous leakage, in particular in the case of fuel injectors having a control valve, which is actuated by way of a piezo actuator, the period of pressure reduction in the high-region of the internal combustion engine, for example in the case of negative load cycles, is sometimes too long.
In contrast to a control valve that is actuated in an electromagnetic manner, the use of a piezo actuator renders it possible to specify the position of the valve actuator in a defined manner.
Below a predetermined pressure value at which the continuous leakage is no longer sufficient for the desired pressure reduction gradient, it is possible to generate an additional switching leakage by means of purposefully controlling the piezo actuator of the injector (part stroke of the control valve). Thus, a pressure reduction gradient is adjusted between the high-pressure region and the low-pressure region, by means of which a pressure reduction can be achieved in the high-pressure region of the internal combustion engine. This method is also described as a LAPD method (Leakage Amplified Pressure Decay).
However, this type of control can no longer be used above a critical pressure value owing to an increased risk of an undesired injection of fuel.
When controlling the injector in the LAPD mode, the restrictor region of the control valve is exploited. The position of the control valve in the part stroke is approached in such a manner that the opened cross section leads to a pressure reduction in the high-pressure region and the pressure reduction in the control chamber of the injector does not exceed the critical value of a force reversal on the injector needle.
However, the purposeful approach and stabilization of a part stroke position of the control valve with a conventional flow/voltage profile for the purpose of controlling the piezo actuator or rather with the known LAPD method is only possible in a limited pressure range (e.g. up to approx. 1200 bar) owing to the elasticity/rigidity of the piezo actuator.